1. Field of the Invention
The present invention relates to methods for removing wrinkles, devices for removing wrinkles, and coating methods. In more particular, the present invention relates to a method for removing wrinkles of a web in a desired zone, particularly in a zone in which a coating liquid is applied thereto, the web being continuously transported by transport means such as guide rollers; to a device for removing wrinkles; and to a method for applying the coating liquid uniformly and stably to the web while wrinkles thereof are appropriately removed.
2. Description of the Related Art
Heretofore, as methods for applying a coating liquid onto a web, for example, there may be mentioned a roll coating method, a gravure coating method, and a doctor coating method. In recent years, an extrusion coating method has drawn attention, in which paint is efficiently used and in which since being applied directly to a web without exposed to the air, the properties of paint are stably maintained.
The extrusion coating method is a coating method in which an extrusion coating head having a front edge surface (located upstream in a web transport direction) and a back edge surface (located downstream in the web transport direction) is pressed toward a web between support rollers or the like, which are used as transport means, so that a predetermined tension is applied to the web and in which the distance between the back edge surface and the web is changed in accordance with the change in amount of a coating liquid extruded from a slit portion so that a film having a predetermined thickness is formed on the web.
In the extrusion coating method described above, due to a tension generated in the longitudinal direction when a web is transported, wrinkles and waves are likely to be formed thereon, and the wrinkles and waves thus formed may cause a continuous or an intermittent variation in thickness in some cases. In particular, in web-coating performed in a process for manufacturing a magnetic recording medium, since the thickness of the web has been decreased (10 μm or less) concomitant with the recent trend toward higher recording density and a larger memory capacity, the stiffness of the web is decreased, and as a result, problems of wrinkles and waves have become more serious. In order to solve the problems described above, a method for transporting a web has become more significant.
As means for preventing influences of wrinkles generated in a coating portion by transport, a backup roller, which is disposed at a position opposing an extrusion coating device with the web provided therebetween, is generally provided. However, by the method described above, since the coating thickness is determined by the distance between the extrusion coating device and the backup roller, in addition to improvement in dimensional accuracy of the backup roller, more strict control of vibration and deflection accuracy has been increasingly demanded, and in particular, it has been very difficult to form a thin film by coating having a thickness of 1 μm or less after drying.
In consideration of the situations described above, a coating device has been disclosed in U.S. Pat. No. 2,578,631 and Japanese Unexamined Patent Application Publication No. 9-141173 as means for obtaining a uniform tension in the width direction of a web at a coating portion, in which tension adjusting means or a compression member is provided in the vicinity of the coating device so as to obtain a uniform tension distribution in the width direction of the web. However, according to the method disclosed in the publications described above, since the adjustment is performed by contacting the compression member or the like directly to the coating portion, the coating portion is largely influenced thereby, and as a result, it has been difficult to determine the process conditions.
In addition, a method has been disclosed in Japanese Unexamined Patent Application Publication No. 11-128822, in which tension adjusting means is pressed to a web so as to generate a uniform tension for removing local elongation and deformation of a web formed in the width direction thereof; part of the web, which is further deformed by the step described above, is recovered by using an inlet roller provided at the coating portion; and subsequently coating is performed. In the method described above, since the coating portion is not directly compressed, the influence on the coating is not significant; however, although sags at the “elongated” portion of the web can be prevented, wrinkles formed in the longitudinal direction of the web by transport cannot be removed. That is, in order to remove the wrinkles of the web in the longitudinal direction formed by the transport, it is important that a force pulling the web outside in the lateral direction be applied thereto at the coating portion so as to remove the wrinkles.
As the mechanism for removing wrinkles, based on the principle described above, for example, techniques disclosed in Japanese Unexamined Patent Application Publication Nos. 5-62182 and 12-197843 may be mentioned. In Japanese Unexamined Patent Application Publication No. 5-62182, a technique has been disclosed in which both sides of a web, which is being transported, are held at guide roll portions in front of and behind a coating portion by rollers and are pressed so that a force in the lateral direction is applied to the web. In addition, in Japanese Unexamined Patent Application Publication No. 12-197843, a technique has been disclosed in which both ends of a web are deformed at a coating portion by a device called an edge lifter, or a force is applied by holding the web with a device having a nip mechanism called a cross guider so that both ends of the web are pulled in the lateral direction. In the techniques described above, of course, the wrinkles formed by the transport of the web are naturally removed; however, when both sides of the web are directly lifted at the coating portion, large wrinkles are disadvantageously formed at bending portions. In addition, when the device directly holds the web and applies a force thereto in the lateral direction, the web has been inevitably damaged thereby.